First and second oil flow passages are defined in a wall portion of a transmission casing. The first oil flow passage has an open end exposed to an interior of the casing. The second oil passage is connected to the first oil passage through a connection opening defined by the first oil passage. A separate pipe member has a predetermined end portion that is inserted into the first oil passage from the open end of the same. The predetermined end portion comprises a cylindrical portion having a fixed length; an end opening defined by an end of the cylindrical portion and exposed to the first oil passage; and an orifice opening defined by a side wall of the cylindrical portion. The orifice opening is blocked by an inner wall of the first oil passage when the connection opening takes a first position relative to the open end of the first oil passage, and exposed to the second oil passage when the connection opening takes a second position relative to the open end of the first oil passage. The second position is closer to the open end of the first oil passage than the first position is.